1. Field of the Invention
This invention relates to coal liquefaction and is particularly concerned with staged temperature hydrogen-donor coal liquefaction.
2. Description of the Prior Art
A number of different processes are being developed for the production of liquid hydrocarbons from coal. Among the most promising of these are processes in which the feed coal is first contacted with a hydrogen-containing gas and a hydrogen-donor solvent at elevated temperature and pressure in a liquefaction reactor and a portion of the liquid product is then catalytically hydrogenated in a solvent hydrogenation reactor to generate a hydrogen-donor solvent for recycle to the liquefaction step. Within the liquefaction zone, the high molecular weight constituents of the coal are cracked and hydrogenated to form lower molecular weight vapor and liquid products. The effluent from the liquefaction reactor is then separated into gases, low molecular weight liquids, and a bottoms stream containing high molecular weight constituents and unconverted mineral matter. The separation of the liquefaction reactor effluent is normally made in such a manner as to produce a bottoms stream consisting of material that boils above about 1000.degree. F. The bottoms stream is composed primarily of high molecular weight hydrocarbons formed when the original high molecular weight coal constituents are only partially converted in the liquefaction reactor, suspended particles of unreacted coal, mineral matter and other solid residues. Depending on liquefaction conditions, the bottoms stream will normally contain from about 40 to 60 wt.% of the high molecular weight hydrocarbons and unconverted coal based on the weight of the original dry coal feed.
It has been suggested that the quantity of high molecular weight bottoms formed from a caking type coal in the above process or similar one-stage liquefaction processes can be decreased and coal conversion therefore increased by utilizing a pretreatment step in which the caking coal is completely dispersed in the hydrogen-donor solvent before the slurry is subjected to liquefaction conditions. Complete dispersion is obtained by maintaining the coal-solvent slurry at a temperature within the range from about 500.degree. F. to about 700.degree. F. as it is agitated. Normally, complete dispersion is indicated when the viscosity of the slurry passes through a maximum and then falls to a value within a predetermined range lower than the maximum. In the agitated pretreatment step an insignificant number of coal molecule bonds are broken and therefore little if any liquefaction actually occurs. The pretreatment step is utilized to obtain a complete dispersion of the coal in the solvent so that the coal particles will be in intimate contact with the hydrogen-donor solvent during the subsequent liquefaction step, and therefore the coal radicals produced in that step can be more effectively stabilized to prevent their recombination. This in turn results in an increased production of low molecular weight liquids.
Although the above-described process supposedly has advantages over other processes for liquefying coals, it has disadvantages in that it is applicable only to the liquefaction of caking type coals, and agitation is required during the pretreatment step. Furthermore, virtually no liquefaction takes place in the pretreatment step.